1. RIVERS
FORM 5
GEOGRAPHY CLASS
2011

2. OBJECTIVES FOR THIS CLASS
THE WATER CYCLE - HOW WATER FLOWS ON REACHING THE SURFACE
PATHWAYS WATER TAKES ON REACHING THE SURFACE
THE DRAINAGE BASIN AND THE RIVER’S DRAINAGE PATTERNS
RIVER PROCESSES: EROSION, TRANSPORTATION AND DEPOSITION
STAGES OF THE RIVER AND THE LANDFORMS AT EACH STAGE

3. WHAT IS A RIVER?
A LARGE NATURAL STREAM OF WATER FLOWING IN A CHANNEL TO THE SEA, A LAKE OR ANOTHER STREAM.

4. SOURCES OF A RIVER? AN UPLAND LAKE A MELTING GLACIER
A SPRING IN AN UPLAND AREA WHERE THE SOIL IS SO SATURATED THAT SURFACE FLOW BEGINS
A SPRING AT THE BOUNDARY BETWEEN PERMEABLE AND IMPERMEABLE ROCKS

5. THE WATER CYCLE IS ALSO KNOWN AS THE HYDROLOGICAL CYCLE.
IT DESCRIBES THE CONTINUOUS MOVEMENT OF WATER ON, ABOVE AND BELOW THE EARTH’S SURFACE.

7. PROCESSES OF THE WATER CYCLE
EVAPORATION – THE PROCESS BY WHICH WATER IS CHANGED INTO WATER VAPOUR (A GAS)
TRANSPIRATION – THE EVAPORATION OF WATER INTO THE ATMOSPHERE FROM THE LEAVES AND STEMS OF PLANTS

8. CONDENSATION – THE PROCESS BY WHICH WATER VAPOUR TURNS INTO WATER DROPLETS. (THIS HAPPENS WHEN WATER VAPOUR IS COOLED) AND CLOUDS ARE FORMED.
PRECIPITATION – THIS OCCURS WHEN WATER IN ANY FORM FALLS FROM THE ATMOSPHERE TO THE SURFACE. (RAIN, HAIL, SLEET, SNOW).

9. PATHWAYS WATER TAKES ON REACHING THE SURFACE
WHEN PRECIPITATION REACHES THE SURFACE IT CAN FOLLOW A NUMBER OF PATHWAYS…
THEY ARE AS FOLLOWS …

10. AN SMALL AMOUNT OF WATER FALLS INTO RIVERS AS DIRECT CHANNEL PRECIPITATION. THE REST FALLS ONTO VEGETATION OR THE GROUND. IF HEAVY RAIN HAS FALLEN PREVIOUSLY THE SOIL IS FULL OF WATER AND IS SAID TO BE SATURATED.

11. BECAUSE THE SOIL IS UNABLE TO TAKE ANYMORE WATER, THE RAIN FLOWS ON THE SURFACE AND THIS IS CALLED SURFACE RUNOFF OR OVERLAND FLOW. IF THE SOIL IS NOT SATURATED, RAINWATER WILL SOAK INTO IT, AND THIS IS CALLED GROUNDWATER FLOW.

12. OTHER KEY DEFINITIONS…
WATER FLOWING THROUGH THE SOIL IS CALLED THROUGHFLOW. INFILTRATION IS THE PASSAGE OF WATER INTO THE SOIL. PERCOLATION IS THE DOWNWARD MOVEMENT OF WATER WITHIN A SOIL OR ROCK. WATER CAN ALSO BE INTERCEPTED (INTERCEPTION) BY THE LEAVES OF PLANTS FROM HITTING THE GROUND.

13. DRAINAGE BASINS
A RIVER’S DRAINAGE BASIN (OR CATCHMENT AREA) IS THE AREA DRAINED BY A RIVER AND ITS TRIBUTARIES.

14. DRAINAGE DENSITY
SOME AREAS HAVE A TIGHTLY PACKED NETWORK OF RIVERS, THAT MEANS IT HAS A HIGH DRAINAGE DENSITY. OTHER AREAS HAVE FEW RIVERS, THAT MEANS THEY HAVE A LOW DRAINAGE DENSITY.

15. WHAT INFLUENCES DRAINAGE DENSITY?
THERE ARE MANY RIVERS WHERE:
RAINFALL IS HIGH
THE ROCKS ARE IMPERMEABLE AND CANNOT ABORB WATER VERY QUICKLY
THERE ARE FEW RIVERS WHERE:
RAINFALL OS LOW
THERE ARE PERMEABLE ROCKS, WHICH CANNOT ABSORB RAINFALL QUICKLY

16. A RIVER’S DRAINAGE BASIN

17. DRAINAGE PATTERNS
RIVERS OFTEN FORM A DISTINCT PATTERN WHICH IS DUE TO THE STRUCTURE OF THE ROCKS IN THE DRAINAGE BASIN.
THERE ARE 3 TYPES OF DRAINAGE PATTERNS RIVERS FOLLOW –
1.DENDRITIC
2.RADIAL
3.TRELLISED

18. DENDRITIC PATTERN
THIS DRAINAGE PATTERN RESEMBLES THE BRANCHES OF A TREE. THE MAIN RIVER IS THE “TRUNK” AND THE OTHER TRIBUTARIES ARE THE “BRANCHES”.THIS PATTERN USUALLY DEVELOPS IN GENTLY SLOPING AREAS

19. RADIAL PATTERN
THIS DRAINAGE PATTERN RESEMBLES THE SPOKES OF A WHEEL. THE RIVERS FLOW OUTWARDS IN ALL DIRECTIONS FROM A CENTRAL POINT. RIVERS ON VOLCANIC CONES USUALLY DEVELOP THIS PATTERN.

20. TRELLSIED PATTERN
THIS DRAINAGE PATTERN RESEMBLES A RECTANGULAR GRID. THIS PATTERN USUALLY DEVELOPS IN AREAS WHER THERE ARE DIFFERENT TYPES OF ROCK (SOME ARE PERMEABLE AND SOME ARE IMPERMEABLE). THE MAIN RIVERS RUN NORTH OR SOUTH AND THE TRIBUTARIES RUN EAST OR WEST(AT RIGHT ANGLES TO THE MAIN RIVER).

22. RIVER PROCESSES EROSION
HYDRAULIC ACTION: THIS IS THE FORCE OF THE RIVER ITSELF REMOVING LOOSE MATERIAL FROM THE BED AND BANKS OF THE RIVER
CORRASION: (OR ABRASION) THIS IS THE PROCESS BY WHICH STONES, SAND GRAINS AND OTHER PARTICLES ARE THROWN AGAINST THE BED AND BANKS TO ERODE THEM

23. ATTRITION:STONES AND ROCKS CARRIED DOWNSTREAM ARE THROWN AGAINST EACH OTHER. THEY ARE WORN AWAY AND OVER TIME BECOME SMALLER.
SOLUTION: RIVER WATER CAN DISSOLVE SOME OF THE MINERALS IN THE ROCKS, ESPECIALLY LIMESTONE

24. TRANSPORT SOLUTION: SOME MATERIALS ARE DISSOLVED IN THE WATER
SUSPENSION: FINE CLAY PARTICLES FLOAT ALONG IN THE RIVER WATER
TRACTION: STONES AND BOULDERS ARE ROLLED ALONG THE RIVERBED.
SALTATION: SAND PARTICLES ARE BOUNCED ALONG THE RIVERBED.

25. DEPOSITION
IF A RIVER DOES NOT HAVE ENOUGH ENERGY AVAILABLE, IT WILL NOT DEPOSIT ITS ENTIRE LOAD AT ONCE. THE HEAVIEST MATERIAL IS ALWAYS DEPOSITED FIRST.

26. LARGE BOULDERS ARE DEPOSITED FIRST
LARGE BOULDERS ARE DEPOSITED FIRST. THEY CAN ONLY BE MOVED WHEN THE SLOPE IS STEEP AND THE RIVER IS FULL OF WATER
GRAVEL, SMALL STONES AND SAND ARE DEPOSITED NEXT.
FINE CLAY PARTICLES SETTLE VERY SLOWLY

27. SILT IS CARRIED ALONG WHEN THE WATER IS MOVING, AND IS DEPOSITED AFTER A FLOOD OR AT THE MOUTH OF THE RIVER.
THE DISSOLVED MATERIAL IS NOT USUALLY DEPSOSITED, BUT IS CARRIED INTO THE SEA WITH THE RIVER WATER.

28. STAGES OF A RIVER
THE UPPER STAGE
THE MIDDLE STAGE
THE LOWER STAGE

29. THE MAIN FEATURES OF THE UPPER STAGE ARE:
A STEEP V-SHAPED VALLEY
A STEEP GRADIENT
INTERLOCKING SPURS
POTHOLES
WATERFALLS, RAPIDS AND GORGES

30. V-SHAPED VALLEYS
IN THE UPPER STAGE MUCH OF THE RIVER’S ENERGY IS NEEDED TO TRANSPORT LOAD. THERE ARE LARGE BOULDERS WHICH CAUSE RAPID EROSION. THIS RESULTS IN THE RIVER CUTTING DOWNWARDS IN ITS BED CAUSING VERTICAL EROSION AND FORMS A V-SHAPED VALLEY.

32. INTERLOCKING SPURS
THE RIVER WINDS ITS WAY (MEANDERS) AROUND OBSTACLES OF HARD ROCK. THIS EVEVTUALLY CREATES SPURS WHICH FORM ON EACH SIDE OF THE RIVER – SO THEY INTERLOCK.

34. POTHOLES
WHERE THE RIVER BED IS UNEVEN, PEBBLES CARRIED BY FAST, SWIRLING WATER, ROTATE IN A CIRCULAR MOVEMENT AND ERODE THE BED FORMING POTHOLES. (JUST LIKE THE POTHOLES IN THE ROADS!)

36. WATERFALLS, GORGES AND RAPIDS
THESE ARE FORMED BY DIFFERENCES IN THE TYPE OF ROCK, AND THE SOFTER ROCK ERODES MORE QUICKLY THAN THE HARDER ROCK … as shown in the following diagram

38. THE MIDDLE STAGE
IN THIS STAGE THE RIVER HAS MANY TRIBUTARIES AND HAS MORE WATER. THIS MEANS IT HAS MORE ENERGY AND CARRIES OUT MORE EROSION. THE RIVER IS WIDENED BY LATERAL EROSION AND HAS THE FOLLOWING FEATURES:
A WIDER V-SHAPED VALEY
MEANDERS

39. MEANDERS
A MEANDER IN GENERAL IS A BEND IN THE WATERCOURSE. A MEANDER IS FORMED WHEN THE MOVING WATER IN A STREAM ERODES THE BANKS AND WIDENS ITS VALLEY.

41. THE LOWER STAGE
THE RIVER CONTIUES TO ERODE DOWNWARDS AND THE VALLEY IS CLOSE TO SEA LEVEL. THE RIVER BECOMES WEAKER BECAUSE THERE IS LESS WATER WHICH MEANS LESS ENERGY. THEREFORE, SEDIMENT IS DEPOSITED.

42. FEATURES OF THE LOWER STAGE ARE:
A VERY WIDE VALLEY
MANY MEANDERS
FREQUENT CHANGES IN THE COURSES OF THE RIVER
A RIVERBED OF SILT AND CLAY – CALLED FLOOD PLAINS
OXBOW LAKES
LEVEES
DELTAS

43. OX-BOW LAKES
An oxbow lake is a U-shaped body of water formed when a wide meander from the main stem of a river is cut off to create a lake.
An oxbow lake is formed when a river creates a meander, due to the river's eroding the bank through hydraulic action and abrasion/corrosion. After a long period of time, the meander becomes very curved, and eventually the neck of the meander will touch the opposite side and the river will cut through the neck, cutting off the meander to form the oxbow lake.

45. FLOOD PLAIN
AS A RIVER FLOWS FROM ITS MIDDLE COURSE TO ITS LOWER COURSE, MEANDERS BECOME EVEN MORE PRONOUNCED, AND THE VALLEY BECOMES WIDER AND FLATTER. WHEN SILT IS DEPOSITED ON THE VALLEY FLOOR MAINLY WHEN THE RIVER FLOODS, THIS RESULTS IN THE FLOOD PLAIN OF THE RIVER.

47. LEVEES
THE RIVER BANK MAY SOMETIMES BE HIGHER THAN THE FLOOD PLAIN. A LEVEE IS A RAISED RIVER BANK BUILT UP FROM ALLUVIAL (SILT) DEPOSITS
HOW ARE THEY FORMED?
AFTER A PERIOD OF RAIN THE RIVER FLOODS AND FLOWS OVER
THE FLOOD CAUSE SILT TO BE DEPOSITED
WHEN THE FLOOD WATERS RECEDE, A THICK DEPOSIT OF MUD IS LEFT AT THE RIVER’S BANKS. AFTER THIS PROCESS HAS BEEN REPEATED A LEVEE IS FORMED.

50. DELTAS
DELTAS ARE FORMED BY THE DEPOSITION OF SEDIMENT AT THE MOUTH OF A RIVER AS IT ENTERS A SEA OR LAKE. DELTAS ONLY FORM UNDER CERTAIN CONDITIONS AND MOST RIVERS DO NOT END IN A DELTA.